Introduction

As previously discussed in part 1 of this article (Mechanical analysis of belt conveyors: how important is it? - Part 1), belt conveyors (BCTs) are uninterrupted material transportation devices made up of a continuous belt that moves on rollers and drums, helping to ensure efficient transportation. These assets are made up of various mechanical elements, such as the drive and tensioning systems.

In addition to the points covered in the first part of the article, the mechanical analysis of conveyors aims to evaluate the mechanical components of the drive system and the stretching system, as well as the impact, load and return rollers. This article presents some of the checks carried out by Kot on these items.

Figure 1: Summary flowchart of mechanical analysis. SOURCE: Kot Collection.

Drive system

The drive system is made up of the motor, coupling and gearbox, and may also contain a flywheel, backstop and brake. In most conveyors, this system provides the power needed to move the belt. In the case of TRs with a regenerative profile (descending), the drive operates as a generator, controlling the speed of the belt and transforming the mechanical energy of the movement into electrical energy.

For both scenarios, the drive system also has the function of starting, stopping and holding the belt by means of brakes and backstops. Figure 2 shows a schematic drawing of a conventional drive system, identifying its main components.

Figure 2: Example of a drive system. SOURCE: Kot Collection.

The power required by the conveyor is also calculated by specialized software used to analyze the mechanical model, as mentioned in article part 1. When sizing and evaluating the components of the drive system, it is essential to know the power required in the different operating conditions that the asset can work in, whether they are common, momentary or unusual.

Based on the powers required, the motor utilization rates for the different operating conditions are evaluated, checking for the possibility of overloads, which can result in unwanted shutdowns or reduce the useful life of the drive system components. The calculation of overloads, whether at the design stage or for the study of existing conveyors, is important information to pass on to the team responsible for analyzing the electrical system.

Gif 1 shows an example of evaluating the power required for loading and unloading material on a long-distance belt conveyor (TCLD). In the study in question, an overload of up to 120% was identified for around 100 seconds during material unloading. This overload was reported to those responsible for sizing the electrical system to take this condition into account.

Gif 1: Evaluation of the power required for loading and unloading material. SOURCE: Kot collection.

The choice of starter type will depend on the specific needs of the conveyor, taking into account factors such as load inertia, motor power and control requirements. In addition, it is important to ensure that the type chosen meets the asset's efficiency and safety standards. The main types of starter are

• Direct start: The conjugate curve of the electric motor is applied (Figure 3A), which generally has a maximum value of more than 200% and a starting current six times higher than the nominal. In this case, the motor starts together with the load (Figure 3B) and, if the start is long, the motor will have a high current for a long time, which can cause breakdowns and reduce its useful life. Therefore, this type of starter is normally used for short TRs, with low power and low load inertia (short starting time).

Figure 3A: Conjugation and current curve of the motor. SOURCE: WEG electronic catalog.

Figure 3B: Conveyor direct start curve. SOURCE: Kot Collection.

• By hydrodynamic coupling: The hydrodynamic coupling decouples the starts of the motor and the load, allowing the motor to do it in a short time, with a high current. The load will start after the coupling fluid starts moving and the power is transmitted between its rotors. In this way, it is possible for this event to occur with a lower overload. It is important to carry out the analysis together with the coupling manufacturer in order to assess the oil temperature during start-up, a fundamental study for the proper sizing of the coupling.
• By frequency inverter: The start is made on a speed curve in a pre-established time and occurs more smoothly, as illustrated in Figure 4. In frequency inverter starts, normally the maximum conjugate supplied by the motor is around 150% of the nominal.

Figure 4: Frequency inverter start curve. SOURCE: https://helixconveyor.com/App/THelp/helpDynTorqueSpeed

In addition to the motor, evaluations are also carried out on the drive's other components: gearbox, couplings, brakes and backstops:

• Gearbox: This item is responsible for reducing the speed of the motor, and mechanical and thermal capacity assessments are carried out. This mechanical analysis relates to the mechanical strength of the gearbox components, while the thermal study involves heat dissipation. If the gearbox is not sized correctly or there is an overload in the drive system, the oil temperature can exceed the maximum permissible. As a result, the viscosity of the oil may decrease, compromising lubrication and creating a scenario conducive to excessive wear of the gear teeth.
• Coupling and backstop: Couplings transmit power between components. The backstop is responsible for preventing the reversal of the belt's direction of rotation, preventing material from returning during idle moments on the conveyor, especially on ascending conveyors, which are more prone to backstopping. The study of these items is carried out in accordance with their manufacturer's definitions, in order to validate their operation and consolidate operational safety.

Figure 5 shows an example of a double drive configuration with backstops on both sides of the drive.

Figure 5: Double drive system with counter recoil on both sides. SOURCE: Kot Collection.

• Brakes: The emergency braking time is evaluated, either statically or dynamically, and the safety factor of the braking torque. Predicting the behavior of the belt and the tensions acting in this condition is fundamental for sizing the conveyor. In addition, it should be noted that dynamic analyses of stops, emergency or otherwise, often identify the need to include a flywheel to soften the dynamic behavior of the belt and control the maximum and minimum stress levels.

In addition to the assessments mentioned for the brakes, thermal analyses are also carried out considering the energy to be dissipated during braking. High brake caliper temperatures can reduce their coefficient of friction with the disc. Therefore, for safe brake operation, it is essential that the necessary heat dissipation is taken into account when sizing the disc and caliper.

Stretching system

Every conveyor has a belt tensioning system, which aims to adjust the tension of the belt along its entire length. Its proper operation is able to prevent non-conformities such as excessive deflection, compression and belt slippage, which impair or make it impossible for the equipment to function. Stretching can be fixed center (screw or hydraulic cylinders), counterweight or winch (electromechanical or hydraulic). Fixed center stretching is normally used for short conveyors, while counterweight and winch stretching is more commonly used for larger TRs. Figure 6 shows a schematic drawing of vertical gravity stretching.

Figure 6: Vertical gravity stretching. SOURCE: Kot Collection.

In the case of vertical or horizontal gravity hoisting, the main checks carried out are on the safety factors of the wire rope and the minimum diameter of the pulleys and sheaves in the system. For electric or hydraulic winch hoisting, the main analyses are of the power required and the winch's braking torque, in addition to the studies carried out for gravity systems.

Finally, the conveyor's stretching stroke is checked according to the belt model, the stretching force, the conveyor's belt tensions in a transient regime and the asset's maintenance needs. An inadequate stroke can cause operational problems, such as collision with the structure and loss of stretching force, as well as hindering maintenance on the conveyor.

Rollers

The rollers are responsible for accommodating the belt along the entire conveyor, which can be impact, load or return. Constant breakage and jamming of the rollers can considerably impair the physical availability of the equipment. Adequate sizing is therefore essential for the asset's efficiency. It should also be noted that locking these components increases the risk of fires.

In short, the service life of the bearings, the shaft deflection in the region of these bearings and the maximum permissible rotation are evaluated, following the recommendations of the applicable standards. Studying the service life of these components and the maximum permissible rotation are important to avoid constant and premature failure of the rollers. On the other hand, the analysis of shaft deflection in the bearing area is important to prevent the rollers from locking. Figure 7 shows the deflection of this shaft schematically.

Figure 7: Deflection of the roller shaft. SOURCE: Kot collection.

Conclusion

Evaluations of the mechanical components of drive systems seek to assess the selection and sizing of the motor, gearbox, couplings, brakes and backstops. Undersizing these components can lead to reduced service life, unscheduled maintenance and even accidents on the asset.

As with the drive system, proper sizing of the stretching system is also fundamental to the functioning and operational safety of conveyors. In addition, rollers are also crucial components for ensuring that the equipment has good availability.

With this in mind, it can be seen that the complete analysis of belt conveyors is extremely important, both for projects and for existing conveyors, whether for sizing systems or for identifying and mitigating problems that arise in the field. Investing in the mechanical analysis of belt conveyors is crucial if your operation is to be more efficient.

Kot has a team of qualified Structural Integrity professionals ready to develop the best engineering solutions for your business. Contact our experts for more information!

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